1. Field of the Invention
The present invention relates to a liquid crystal display and a back light to be arranged behind a liquid crystal panel in the liquid crystal display.
2. Description of the Related Art
Generally, liquid crystal displays have a liquid crystal panel and a back light for irradiating the backside of the liquid crystal panel with light. In recent years, liquid crystal displays have improved and approached CRTs (Cathode Ray Tubes) in terms of performance. Liquid crystal displays thus have found an increasing range of applications, including navigation systems to be mounted on motor vehicles.
In motor vehicles, the interior brightness varies greatly between daytime and nighttime hours. Cars are dark inside in the nighttime, so that cars' navigation systems must be lowered sufficiently in screen brightness in the nighttime. That is, liquid crystal displays for use in the navigation systems need to have a smaller minimum luminance for the sake of nighttime use.
FIG. 1 shows a block diagram of a control circuit in a back light 10 to be used for this type of liquid crystal display. In the diagram, the back light 10 includes an oscillating circuit 12, lighting circuits 14a, 14b, and 14c, a light volume adjusting circuit 16, and fluorescent tubes 18a, 18b, and 18c. 
The light volume adjusting circuit 16 receives brightness adjusting input which is generated in accordance with a luminance adjusting signal from exterior, and outputs, to the oscillating circuit 12, a light volume adjusting signal for adjusting the fluorescent tubes 18a, 18b, and 18c in brightness. The oscillating circuit 12 generates an alternating voltage corresponding to the light volume adjusting signal out of the power supplied from a power source, and outputs the generated alternating voltage to the lighting circuits 14a, 14b, and 14c. The lighting circuits 14a, 14b, and 14c boost the alternating voltage output from the oscillating circuit 12, and supply the boosted voltages to the fluorescent tubes 18a, 18b, and 18c. The fluorescent tubes 18a, 18b, and 18c light up at luminances corresponding to the voltage waveforms supplied.
FIG. 2 shows an overview of structure of a light emitting part 10a in the back light 10. The light emitting part 10a has a reflector 20 for accommodating the fluorescent tubes 18a, 18b, and 18c. 
The inner surface of the reflector 20 is given a reflecting coat of metal. The fluorescent tubes 18a, 18b, and 18c are arranged in parallel inside the reflector 20. The light emitted from the fluorescent tubes 18a, 18b, and 18c radiates out directly or after reflected from the inner surface of the reflector 20.
In the back light 10 shown in FIG. 1, the single light volume adjusting circuit 16 adjusts the plurality of fluorescent tubes 18a, 18b, and 18c in luminance. On this account, when the back light 10 produces an output of the minimum luminance, the fluorescent tubes 18a, 18b, and 18c are lit at their respective minimum luminances. Consequently, the minimum luminance possible for the back light 10 to output is the sum of the minimum luminances of the individual fluorescent tubes 18a, 18b, and 18c. When liquid crystal displays having such a back light are applied to the navigation systems, the screen brightness cannot be lowered to an appropriate brightness in nighttime use.
According to the structure of the light emitting part 10a of the back light 10 shown in FIG. 2, the central fluorescent tube 18b faces a smaller area of the reflector 20 and the outer fluorescent tubes 18a and 18b face greater areas of the reflector 20. The parasitic capacitance occurring between the fluorescent tube 18b and the reflector 20 is therefore smaller than the parasitic capacitance occurring between the fluorescent tube 18a and the reflector 20, and the parasitic capacitance occurring between the fluorescent tube 18c and the reflector 20.
Therefore, the current to flow through the fluorescent tube 18b is greater than the currents to flow the fluorescent tubes 18a and 18c. This shortens the life of the fluorescent tube 18b more than the lives of the fluorescent tubes 18a and 18c. In general, fluorescent tubes of a back light cannot be replaced separately. Thus, the entire back light must be replaced when any one of the fluorescent tubes no longer works. That is, the life of a back light becomes shorter depending on the fluorescent tube of the shortest life.
Recently, parts of the liquid crystal displays tend to get smaller in size due to a growing demand for liquid crystal panels of larger size. The reflectors accommodating the fluorescent tubes of the back lights also have the inclination to shrink in size. This results in reducing interior spaces of the reflectors and easy trapping of heat within the reflectors. Consequently, if a plurality of fluorescent tubes is used, there is a possibility that concentrate generated heat therein may hamper sufficient heat dissipation.